This invention relates to process and apparatus for the manufacture of oriented plastics tubes, and in particular to the manufacture of tubes which have been stretched (drawn) in both the circumferential and axial directions to orient the material in the direction of stretching and enhance its mechanical properties.
International Patent Application No. WO 90/02644 describes one process for the manufacture of thermoplastics tubes for example for unplasticised polyvinyl chloride (uPVC) which have a degree of orientation in the circumferential direction that improves properties such as resistance to hoop stresses, and renders the tubes particularly suitable for transmission of water under pressure. The process described in that patent application comprises:
(i) extruding a tube of plastics material; PA1 (ii) temperature conditioning the extruded tube to bring it to a temperature suitable for expansion; PA1 (iii) diametrically expanding the tube by application of an internal pressure to the tube that is limited at its downstream end by a plug that is inflatable or otherwise expandable to maintain pressure within the expansion zone; and PA1 (iv) cooling the expanded tube to set the tube in its diametrically expanded configuration.
To pull the tube through this process line a first haul-off tractor is provided before the temperature conditioning zone and another haul-off tractor is provided downstream of the expansion and cooling zones. Axial draw may be introduced into the product by running the downstream tractor at a higher haul-off speed than the first.
Other processes have been proposed in which the diametrical expansion is achieved by drawing the tube over a solid mandrel. The present invention is applicable also to such processes.